Cyclin is a general term to describe polypeptides that are subunits controlling the activities of cyclin-dependant protein kinase (Cdk) and eight species of cyclin, namely cyclins A-H, have been documented. Cyclin is known to form a complex with Cdk and to exhibit the capability of intracelluar phosphorylation.
Also, structural characteristics common in cyclins are that they passes a region called “cyclin box” which comprises about 100 amino acids within portions of their amino acid sequences. It is recognized that the eight species of cyclins hitherto known are provided with a high degree of homology in the amino acid sequences of this cyclin box. Hence, this cyclin box portion is believed to be a step necessary for binding to Cdk and controlling Cdk.
Furthermore, it is also recognized that the ability of cyclins to phosphorylate Cdk plays a critical role in the control of cell proliferation and through their ability cyclins bear close relation to phenomena such as cancer and immunity. Also, it is suggested that some cyclins are widely involved not only in the control of cell cycle, but also in the signal transmission.
Accordingly, there is strong likelihood that proteins having a high degree of homology in the amino acid sequence for the region called “cyclin box” as described above are cyclins. In this case, it is therefore anticipated that the proteins have the binding ability to Cdk and further that they have the ability to control kinase. See, Experimental Medicine, vol. 13, No. 6 (special issue), 1995.